The present invention relates to the processing of mixed metal oxides in forms which combine high macroporosity and high mechanical strength.
It is desirable in many industrial applications, including flue gas desulfurization, coal gas or other fuel gas cleanup processes at high temperatures, as well as certain catalytic reactions such as fuel conversion and hydrodemetallation, to utilize solid absorbents or catalysts which have "open pore" structures, (i.e., a predominance of macropores). Such open pore structures present little or no resistance to gas diffusion through the pores, are less prone to pore-mouth blocking, and thereby allow better utilization of the solid For any practical application involving the use of solid-gas contactors (such as packed-bed, fluidized-bed or moving-bed reactors) the solids should be used in forms which are mechanically strong and resistant to attrition. Otherwise, the result will be a large pressure drop through the reactor, particulate matter elutriation and clogging of valves or other components of piping lines. Additionally, most reaction systems would benefit from the use of relatively high surface area solids which would enhance reaction kinetics.
Bulk mixed oxide compounds are typically prepared by the solid compounding technique. This involves several cycles of powder mixing and firing at high temperatures (usually higher than 900.degree.-1000.degree. C.) to effect the solid state reaction. The mixed oxide powder thus prepared is further processed conventionally by mixing with inorganic and organic additives and water followed by extrusion or pelletization, drying and heat treatment. These materials are often sintered, i.e., they consist of large grains with low surface area, and have relatively low pore volumes in order to preserve high mechanical strength. If a large amount of organic additive is used to create higher porosity during the heating step, the pore volume of the material increases, albeit at the expense of the compressive crush strength.
In the area of fuel gas cleanup at high temperatures, regenerable mixed oxide sorbents that combine relatively high surface area, high pore volume and good mechanical strength are needed to meet the requirements of cyclic high-temperature operation at high efficiency and with high sorbent utilization. Recent work in the area of hot gas cleanup has identified several regenerable mixed oxide compounds and oxide solid solutions that can effectively remove H.sub.2 S in the temperature range of 1000.degree.-1300.degree. F. See, U.S. Pat. No. 4,729,889; Flytzani-Stephanopoulos, M. et al. (1985), "Novel Sorbent for High-Temperature Regenerative H.sub.2 S Removal," Final Report DOE/MC/20417-1898; and Tamhankar S.S., et al. (1986), Ind. Eng. Chem. Pros. Des. and Dev., 25, 429-437. In that work, the sorbents (in the form of granules, -20 +35 mesh) were prepared with high surface area and macroporosity according to the amorphous citrate technique. These properties were shown to lead to fast absorption and regeneration rates, high sorbent utilization, and good regenerability.
Despite this work there remains a need to prepare strong absorbents and catalysts in practical forms, i.e., greater than 1-2 mm size, which have desired levels of macroporosity and surface area. Accordingly, it is an object of the invention to provide a method of preparing metal oxide and mixed metal oxide catalysts and absorbents which combine desired levels of surface area with high macroporosity and high mechanical strength It is also an object of the invention to provide a more simplified method of preparing such catalysts and absorbents while controlling the physical properties of these materials. Another object of the invention is to provide strong, macroporous extruded single oxide and mixed oxide compositions which are useful as absorbents and catalysts. Other objects of the invention will become apparent to those skilled in the art upon reading the present disclosure.